1. Field of Invention
This invention relates to a transmitter-receiver of an ultrasonic distance measuring device which transmits an ultrasonic wave toward a surface to be measured and receives the ultrasonic wave reflected therefrom to measure the distance to the surface of measurement on the basis of, for example, the time interval from the transmission of the ultrasonic wave to the reception thereof.
2. Description of the Prior Art
FIG. 1 shows a conventional ultrasonic distance measuring device comprising a cylindrical piezoelectric vibrator 1a, made of a ceramic material; a plastic casing 2a, which also serves as an acoustic matching layer; a damping material 3a, for damping the piezoelectric vibrator 1a; and a reflector 4a, for changing the course of ultrasonic waves transmitted from and received by piezoelectric vibrator 1a to impart directivity to the ultrasonic waves.
The device operates as follows:
1. Transmission of ultrasonic waves.
When electric pulses are applied to piezoelectric vibrator 1a, vibrator 1a causes breathing vibrations in the radial direction, resulting in an ultrasonic wave which travels through the air in a normal direction with respect to the outer peripheral portion of piezoelectric vibrator 1a. This ultrasonic wave has its course changed by reflector 4a to travel downward (as viewed in FIG. 1) in the form of a doughnut shaped beam, as shown in FIG. 2.
2. Reception of ultrasonic waves.
The ultrasonic wave which enters the ultrasonic transmitter-receiver from the bottom (as viewed in FIG. 1) is converged by reflector 4a so as to pass through casing 2a and apply a stress to the outer peripheral portion of piezoelectric vibrator 1a. Vibrator 1a generates an electric field between electrodes which has an intensity corresponding to the level of the applied stress.
Examples of practical uses of the ultrasonic transmitter-receiver employing the above principles include a distance measuring device, a level meter, etc. In such devices, the following properties are required.
1. High damping characteristics.
An ultrasonic distance measuring device is an instrument wherein an ultrasonic wave is radiated toward an object and the interval of time t from the transmission of the ultrasonic wave to the arrival of the wave reflected from the object is measured to obtain the distance L to the object according to the following equation. EQU L=1/2ct (1)
wherein c is the sound veloicty in the propagation medium and t is as above defined.
However, since vibrator 1a is of ceramic and generally has a large inertia, even after the electric drive pulses shown in FIG. 3(A) have disappeared, attenuating vibration b (hereinafter referred to as "residual vibration b") continues, as shown in FIG. 3(B). For this reason, when the distance to the object of measurement is short, the reflected wave c may arrive at the transmitter-receiver while the residual vibration b still remains, as shown in FIG. 3(C), resulting in impossibility to separate and discriminate the two waves from each other. In other words, it is difficult to measure a point blank range.
Conversely, when the distance to an object of measurement is great, the amplitude of the reflected wave is extremely small as shown in FIG. 3(D). Therefore, it is necessary to amplify the voltage of the received signal to a substantial extent. Since in this simplification the electromotive force relative to the residual vibration b is also amplified, it is necessary, in order to prevent detection of the residual vibration b, to provide a prohibition region (called "dead zone") e so that detection of the reflected wave c is prohibited until the amplitude of the residual vibration b becomes smaller than that of the reflected wave c, as shown in FIG. 3(E).
Under those circumstances, if the distance measurement device is designed to measure long distances, it becomes impossible to measure short distances. On the other hand, if the device is designed to be capable of measuring short distances (i.e. to shorten the dead zone), it becomes difficult to measure long distances at which the amplitude of the reflected wave is relatively small.
In order to avoid these problems, it is the general practice to damp the piezoelectric vibrator 1a by means of damping material 3a as shown in FIG. 1. This conventional practice, however, involves the following problems.
1-A. It is considerably difficult to select a damping material which exhibits a satisfactory or appropriate damping effect over a wide temperature range. More specifically, most viscous substances change their physical properties to a substantial extent in accordance with temperature. Thus, it is, actually, not easy to find a damping material which exhibits the necessary and sufficient damping characteristics over a wide temperature range, from low temperature to high temperature.
1-B. To apply damping by means of a damping material is to suppress vibration. This leads to a reduction in the sound pressure at the time of transmission.
2. High-efficient transmission and reception characteristics.
It is preferable that transmission and reception of sound be effected with a minimum of electric power from the view point of energy conservation and safety.
The FIG. 1 device, however, suffers from the following problems in this respect.
2-A. Since damping is applied by means of the damping material 3a, it is necessary, in order to obtain a desired amplitude of vibration generated by the vibrator, to apply a considerably larger driving power than in the case of the piezoelectric vibrator 1a alone, for example, 10 times, i.e. about 1 kV in terms of voltage.
2-B. Since the acoustic impedance (ec, wherein e is density, and c is sound velocity) of the vibrator 1a is about 5 orders of magnitude larger than the acoustic impedance of air, the energy, which is transmitted into air when vibrator 1a vibrates, is extremely small. In order to overcome this problem, it is conventional practice to release sound into air through the casing 2a, serving as an acoustic matching layer, made of a plastic material which is acoustically softer (smaller in terms of acoustic impedance ec) than the vibrator 1a. When the thickness of layer 2a is set at a quarter wavelength, the energy propagation efficiency reaches its maximum. However, if the sound velocity relative to the plastic material used changes with temperature change, the equivalent thickness of the layer shifts from the quarter wavelength, resulting in the lowering of the sound propagation efficiency. The presence of layer 2a leads to a change of transmission and reception characteristics with temperature change.
3. Simple structure.
As just described, the conventional device of FIG. 1 needs associated elements such as damping material 3a and matching layer 2a in addition to the vibrator 1a, in order to bring the characteristics close to the ideal. As a result, the characteristics are greatly affected by these elements with respect to temperature change or the like. Ideally, it is preferable to eliminate the need for these extra elements.